In the context of the present application, the term turbid medium is to be understood to mean a substance consisting of a material having a high light scattering coefficient, such as for example an Intralipid solution or biological tissue. Further, light is to be understood to mean electromagnetic radiation of a wavelength in the range from 400 nm to 1400 nm. The term “an element is optically coupled to another element” means that at least one light path is formed along which light is transmitted between the elements. The term “optical properties” covers the reduced scattering coefficient μ's and the absorption coefficient μa. Furthermore, “matching optical properties” is to be understood as having a similar reduced scattering coefficient μ's and a similar absorption coefficient μa.
In recent years, several methods and devices for examining turbid media, e.g. female breast tissue, have been developed. In particular, new devices for detection and analysis of breast cancer have been developed and existing technologies have been improved. Several types of devices for imaging the interior of a turbid medium by use of light have been developed. Examples for such devices are mammography devices and devices for examining other parts of human or animal bodies. A prominent example for a method for imaging the interior of a turbid medium is Diffuse Optical Tomography (DOT). In particular, such devices are intended for the localization of inhomogeneities in in vivo breast tissue of a part of a breast of a female human body. A malignant tumor is an example for such an inhomogeneity. The devices are intended to detect such inhomogeneities when they are still small, so that for example carcinomas can be detected at an early stage. A particular advantage of such devices is that the patient does not have to be exposed to the risks of examination by means of ionizing radiation, as e.g. X-rays.
New approaches for further enhancing the accuracy of methods for detecting breast cancer by use of light have been made. For example, a fluorescent dye has been developed which can be injected into the body and will accumulate in cancer cells. If this fluorescent dye then becomes excited with light of a suitable wavelength, the locally emitted light can be detected. Based on the emitted light, size and localization of carcinoma can be determined. Thus a powerful method for detection and localization of breast cancer is provided.
WO 00/56206 A1 discloses a device for imaging the interior of a turbid medium by using a light source to irradiate the turbid medium and photodetectors for measuring a part of the light transported through the turbid medium. A control unit is provided for reconstructing an interior of the turbid medium on the basis of the measured intensities. The disclosed device is particularly adapted for examining female breasts. In order to allow the examination of the turbid medium, the device is provided with a receptacle enclosing a measuring volume and arranged to receive the turbid medium. The light used for examining the turbid medium has to be transmitted from the light source to the turbid medium and from the turbid medium to the photodetectors. Due to different sizes of the turbid media to be examined, the size of the receptacle for receiving the turbid medium does not perfectly match the size of the turbid medium, i.e. a space remains between the receptacle and the turbid medium. A number of light paths coupling to the light source and a number of light paths coupling to photodetectors are distributed across the wall of the receptacle, i.e. ends of optical fibers acting as light guides are connected to the wall of the receptacle. The space between the receptacle and the turbid medium is filled with a so-called optically matching fluid as an optically matching medium. The optically matching fluid provides optical coupling between the part of the turbid medium to be imaged and the light guides connecting to the light source and the photodetectors, respectively. Further, the optically matching fluid is intended to prevent optical short-cutting between the light source and the photodetectors, i.e. light transmitted from the light source to the photodetectors without being transmitted through the turbid medium. The optically matching fluid counteracts boundary effects in the reconstructed image which are caused by the difference in optical contrast between the interior of the turbid medium in the receptacle and the remaining space in the receptacle. In the disclosed device, the positions of the photodetectors and of the light source relative to the receptacle, and thus relative to the examined turbid medium, are fixed. The light source subsequently irradiates the turbid medium and the photodetectors measure a part of the light transmitted through the turbid medium. A plurality of such measurements are performed and, based on the results of the measurements, the control unit reconstructs the image of the examined turbid medium.
According to the prior art implementation of such a device for imaging the interior of a turbid medium, the light enters and leaves the receptacle via fibers which are at fixed locations. In order to perform a complete examination of the turbid medium, the light has to be subsequently directed to the turbid medium from different directions and the corresponding signals of the detectors have to be detected. Directing light through the different fibers for generating a complete image of the examined turbid medium requires a large, heavy, and expensive fiber switch which makes the overall system design expensive. Further, in the known implementation, due to the switching of the different fibers, the distance between the end at the receptacle side of the fiber connecting to the light source and the end at the receptacle side of a fiber connecting to a particular detector varies during the scan. Thus, during a scan, the end of the corresponding fiber of each detector is sometimes very close to the end of the fiber connecting to the light source and sometimes far away from it. As a consequence, the strength of the signal received by a particular detector varies over a large range. In order to cope with this situation, the detectors have to cope with a large dynamic range of received signal strengths which makes the required detectors expensive and difficult to calibrate. Further, in use in diffuse optical fluorescence tomography using a fluorescent contrast agent the signal levels are very low, especially when the optical path through the tissue is long. This necessitates the use of sensitive low-noise detectors which are expensive. Since a large number of detectors is required in the known devices for providing the required image—for example about 250 detectors in a device known to the applicant—achieving a high accuracy image becomes expensive.